The present invention relates to a perfected air pad comprising a body having two opposite faces. More specifically, the present invention relates to an air pad for supporting the work table of a measuring machine or machine tool, e.g. a printed circuit drilling machine.
Known pads of the aforementioned type are widely employed on the slideways of mutually mobile members, by virtue of the low friction coefficient and the straightforward design of the pads. The precision of the slideway depends on the size of the gap between the respective pad and slideway surfaces, which gap decreases in size alongside an increase in the load on the pad. In the presence of light loads and low air supply pressure, even small variations in load result in a considerable variation in the size of the gap, thus impairing the precision of the slideway.
For applications in which the pads are used for supporting the work table of machine tools or measuring machines, which normally feature an air pressure of 2-3 bar, attempts have been made to increase the specific load by constructing extremely heavy tables, which are invariably expensive by virtue of requiring extremely powerful drive means. Even in the case of massive work tables, however, when accelerating or decelerating, the load on the pads varies enormously and differently from one pad to another. The load on the temporarily relieved pads therefore falls drastically, thus varying the gap and resulting in vibration of the table. Further vibration may also be caused by the work holder and/or tool engaging the work.
One design proposal for eliminating such vibration is to connect each air pad to a vibration damper operated solely when accelerating or decelerating. Such pads, however, fail to provide for compensating for variations in load caused by other factors, such as the action of the tool on the work.
Yet another proposal is for a work table which is moved along two axes by means of an intermediate plate made of granite and in turn moving in relation to a fixed slideway also made of granite. An extensive portion of each face of the intermediate plate presents a number of supporting orifices cooperating with a number of vacuum elements for creating a certain preload on the slideway surfaces.
Vacuum preloading as described above presents several drawbacks. Firstly, the preload achievable is relatively low, thus providing for only very limited compensation of the variation in load. Secondly, the number of vacuum elements is necessarily limited, by virtue of requiring a large-section chamber and being located on the same face as the supporting orifices. Finally, the vacuum elements require a number of conduits connected to a vacuum source and distinct from those of the supporting orifices. As such, the supporting system is not only ineffective, but also highly complex and expensive.